Showing papers on "Amplitude damping channel published in 1995"

Dense Coding Based on Quantum Entanglement

Abstract: We analyse the Bennett-Wiesner dense quantum coding scheme for non-maximally entangled carriers of information. Our generalization shows that the detection scheme proposed by Bennett and Wiesner is optimal even when the entanglement is not perfect. We comment on the optical realization of the scheme.

Inhibition of the measurement of the wave function of a single quantum system in repeated weak quantum nondemolition measurements.

Abstract: It is shown that a series of repeated weak quantum nondemolition measurements performed on a single quantum system gives no information about the wave function of the system. The physical explanation, based on the quantum Brownian motion and the continuous collapse of the wave function which originate in the projection postulate, is discussed in two specific examples.

Quantum theory of activated rate processes: A maximum free energy approach

Abstract: A quantum theory of activated rate processes applicable to nonlinear potentials of interaction is developed. The central premise is that the rate is determined by the point of maximal quantum free energy separating reactants and products. The quantum free energy is defined in terms of a quantum centroid potential. The resulting rate expressions reduce to known limits for generalized Langevin equations and their Hamiltonian representation. They also reduce in the classical limit to previous results derived using an optimal planar dividing surface classical variational transition‐state theory. A saddle‐point estimate of the quantum rate leads to a generalization of Wolynes’ high temperature rate expression valid for nonlinear system bath interactions and anharmonic baths. Maximizing the free energy leads to a quantum friction function. Application to realistic systems demands the computation only of centroid densities.

Quantum bound states in a double-bend quantum channel

Abstract: By use of the mode‐matching technique the quantum bound states in a double‐bend quantum channel of finite length connecting to two 2D electron gas reservoirs have been investigated in detail. The conductance G of the quantum system is calculated as a function of Fermi energy and the electron density associated with bound states. It is found that there exists one resonant peak in G corresponding to resonant tunneling via one quasibound state below the first conductance plateau for the quantum channel with double‐bend continuity and two resonant peaks in G corresponding to resonant tunneling via two quasibound states which are symmetric and antisymmetric superposition of two local bound states localized at two right‐angle bends below the first conductance plateau for the quantum channel with double‐bend discontinuity. At finite temperature the results are compared with experimental results and are found to explain them well.

Lower bound for mutual information of a quantum channel.

Abstract: We obtain a lower bound for the mutual information of a quantum transmission channel, which is perfectly analogous with Holevo's upper bound. The Uhlmann inequality for relative entropies is used, in a reverted order, for a completely positive mapping related with the mixed coherent states introduced by us seventeen years ago. Possible applications to quantum cryptography are discussed.

Quantum cryptography in noisy channels

Abstract: We provide a complete proof of the security of quantum cryptography against any eavesdropping attack including coherent measurements even in the presence of noise. Polarization-based cryptographic schemes are shown to be equivalent to EPR-based schemes. We also show that the performance of a noisy channel approaches that of a noiseless one as the error rate tends to zero. (i.e., the secrecy capacity $C_s (\epsilon) \to C_s (0)$ as $\epsilon \to 0$.) One implication of our results is that one can {\it double} the efficiency of a most well-known quantum cryptographic scheme proposed by Bennett and Brassard simply by assigning vastly different probabilities to the two conjugate bases.

Capacity of a memoryless quantum communication channel

Abstract: We introduce a proper framework of coding problems for a quantum memoryless channel and derive an asymptotic formula for the channel capacity having an operational significance. Some general lower and upper bounds for the quantum channel capacity are also derived.

Quantum Transport Modeling of Mesoscopic Devices: Application of Wigner Distribution Function

Abstract: The dynamic and nonlinear quantum transport in mesoscopic devices, especially, in a quantum wire and a quantum well laser are studied based on the Wigner function model. In the simulation of a quantum wire, the contacts to the quantum wire are modeled carefully. It is found that in the nonlinear transport regime, the space charge significantly affects the current-voltage characteristics of the quantum wire. Further, the dynamic responses of the quantum wire are studied when the bias voltage is switched abruptly. In the simulation of a quantum well laser, the bipolar quantum transport is discussed by solving the three Liouville equations for electron, heavy-hole and light-hole simultaneously. The bottleneck phenomenon of carrier injection into the multi-quantum wells is discussed.

Quantum Interference, State Engineering, and Quantum Eraser

Abstract: To summarize in one pregnant sentence the most recent result has always been and still is the requirement every scientist has to fulfill when he or she wants to explain his or her research to John A. Wheeler, the great scientist, deep philosopher, and extraordinary human being we honor at this conference. The central lesson of quantum mechanics, that is, interfering transition probability amplitudes rather than probabilities, stands out nowhere clearer than in the double-slit experiment, the center of the long-standing debate between Bohr and Einstein.I Nobody has identified this simple, but far-reaching difference between classical and quantum physics as the origin of so many different physical phenomena ranging from nuclear physics2 via rainbow scattering3s4 to squeezed state physics5 as John A. Wheeler. Nothing is more appropriate at a conference celebrating this great man than to present yet another illustration of this one-sentence summary-path information implies probabilities, whereas no path information implies interfering probability amplitudes. In this article, we transfer the double-slit configuration into the rather abstract space of atomic excitation and show how by erasing “which path” information&Il we can generate quantum coherence and even d o state engineering,12-26 that is, manipulate quantum states in a controlled way. Our quantum eraser is spontaneous emission together with rcabsorption in a detector. The article is organized as follows: In the second section, we send two excited two-level atoms, one at a time, through a cavity and find noninterfering contributions to the photon statistics of the cavity field because we still have the information of which atom has deposited a photon in the cavity. We erase this information via spontaneous emission and reabsorption of the emitted photon and obtain interfering parts in the photon statistics of the cavity field. In the third section, we first extend our investigations from two two-level atoms to two three-level atoms and then to many three-level atoms. In the fourth section, we show that we can use the scheme of the third section in order to manipulate the state of the cavity field in a controlled way; that is, wc can d o quantum state engineering. In the fifth section, we give a brief summary of our investigations.

Inhibition of mixing in chaotic quantum dynamics

Abstract: We study the quantum chaotic dynamics of an initially well-localized wave packet in a cosine potential perturbed by an external time-dependent force. For our choice of initial condition and with [h bar] small but finite, we find that the wvae packet behaves classically (meaning that the quantum behavior is indistinguishable from that of the analogous classical system) as long as the motion is confined to the interior of the remnant separatrix of the cosine potential. Once the classical motion becomes unbounded, however, we find that quantum interference effects dominate. This interference leads to a long-lived accumulation of quantum ampliutde on top of the cosine barrier. This pinning of the amplitude on the barrier is a dynamic mechanism for the quantum inhibition of classical mixing.

Quantum bound states in a ballistic quantum channel with a multiple double-bend discontinuity

Abstract: We have investigated in detail the quasibound states of a quantum channel with a multiple double-bend discontinuity. We assume that our quantum system is of finite length and connected to two 2DEG reservoirs. The mode-matching technique has been employed. We have predicted that there are 2N resonant peaks of the conductance below the first transverse-mode energy for the quantum channel with an N double-bend discontinuity, which means the formation of one-electron miniband structure below the first transverse-mode energy for a lateral superlattice consisting of finite periods of double-bend structure. Our results clearly indicate that the 2N quasibound states of the channel with energies below the first transverse-mode energy are 2N-fold splitting quasibound states due to the coupling between the local bound states. Our analytical results from the nearest interactive model and our numerical results are qualitatively well consistent.

Quantum transport for a many-body system

Abstract: We treat the quantum transport of an interacting system of electrons, impurities and phonons. The dynamic system is in a time-dependent electric field. We analyzed quantum transport process by using the quantum generalized Langevin equation, in which the system is shown to be equivalent to a quantum particle in a heat bath. After eliminating the heat-bath variables, the equation of motion for the quantum particle is written in a form of quantum generalized Langevin equation, with a memory term which reflects the retarded effects of the heat bath on the quantum particle.